Swelling-activated K+ fluxes in vascular endothelial cells: role of intracellular Ca2+

Cell swelling activates cloned Ca(2+)-activated K(+) channels: a role for the F-actin cytoskeleton

Cloned Ca(2+)-activated K(+) channels of intermediate (hIK) or small (rSK3) conductance were expressed in HEK 293 cells, and channel activity was monitored using whole-cell patch clamp. hIK and rSK3 currents already activated by intracellular calcium were further increased by 95% and 125%, respectively, upon exposure of the cells to a 33% decrease in extracellular osmolarity. hIK and rSK3 currents were inhibited by 46% and 32%, respectively, by a 50% increase in extracellular osmolarity. Cell swelling and channel activation were not associated with detectable increases in [Ca(2+)](i), evidenced by population and single-cell measurements. In addition, inhibitors of IK and SK channels significantly reduced the rate of regulatory volume decrease (RVD) in cells expressing these channels. Cell swelling induced a decrease, and cell shrinkage an increase, in net cellular F-actin content. The swelling-induced activation of hIK channels was strongly inhibited by cytochalasin D (CD), in concentrations that caused depolymerization of F-actin filaments, indicating a role for the F-actin cytoskeleton in modulation of hIK by changes in cell volume. In conclusion, hIK and rSK3 channels are activated by cell swelling and inhibited by shrinkage. A role for the F-actin cytoskeleton in the swelling-induced activation of hIK channels is suggested.

Hyposmotic shock: effects on rubidium/potassium efflux in normal and ischemic rat hearts, assessed by 87Rb and 31P NMR

The study evaluated effects of hyposmotic shock on the rate of Rb(+)/K(+) efflux, intracellular pH and energetics in Langendorff-perfused rat hearts with the help of 87Rb- and 31P-NMR. Two models of hyposmotic shock were compared: (1) normosmotic hearts perfused with low [NaCl] (70 mM) buffer, (2) hyperosmotic hearts equilibrated with additional methyl alpha-D-glucopyranoside (Me-GPD, 90 or 33 mM) or urea (90 mM) perfused with normosmotic buffer. Four minutes after hyposmotic shock, Rb(+) efflux rate constant transiently increased approximately two-fold, while pH transiently decreased by 0.08 and 0.06 units, in the first and the second models, respectively, without significant changes in phosphocreatine and ATP. Hyposmotic shock (second model) did not change the rate of Rb(+)/K(+) uptake, indicating that the activity of Na(+)/K(+) ATPase was not affected. Dimethylamiloride (DMA) (10 microM) abolished activation of the Rb(+)/K(+) efflux in the second model; however, Na(+)/H(+) exchanger was not involved, because intracellular acidosis induced by the hyposmotic shock was not enhanced by DMA treatment. After 12 or 20 min of global ischemia, the rate of Rb(+)/K(+) efflux increased by 120%. Inhibitor of the ATP-sensitive potassium channels, glibenclamide (5 microM), partially (40%) decreased the rate constant; however, reperfusion with hyperosmolar buffer (90 mM Me-GPD) did not. We concluded that the shock-induced stimulation of Rb(+)/K(+) efflux occurred, at least partially, through the DMA-sensitive cation/H(+) exchanger and swelling-induced mechanisms did not considerably contribute to the ischemia-reperfusion-induced activation of Rb(+)/K(+) efflux.

Signaling events during swelling and regulatory volume decrease

Brain cell swelling compromises neuronal function and survival by the risk of generation of ischemia episodes as compression of small vessels occurs due to the limits to expansion imposed by the rigid skull. External osmolarity reductions or intracellular accumulation of osmotically active solutes result in cell swelling which can be counteracted by extrusion of osmolytes through specific efflux pathways. Characterization of these pathways has received considerable attention, and there is now interest in the understanding of the intracellular signaling events involved in their activation and regulation. Calcium and calmodulin, phosphoinositides and cAMP may act as second messengers, carrying the information about a cell volume change into signaling enzymes. Small GTPases, protein tyrosine kinases and phospholipases, also appear to be part of the signaling cascades ultimately modulating the osmolyte efflux pathways. This review focus on i) the influence of hyposmotic and isosmotic swelling on these signaling events and molecules and ii) the effects of manipulating their function on the osmolyte fluxes, particularly K+, CI- and amino acids, and on the consequent efficiency of cell volume adjustment.

Influence of increase in osmotic pressure with sucrose on relaxation and cyclonucleotides levels in isolated rat aorta

The influence of increases in osmolarity by addition of sucrose were investigated on relaxation and changes in adenosine 3':5'-cyclic monophosphate (cyclic AMP) and guanosine 3':5'-cyclic monophosphate (cyclic GMP) levels in isolated rat aortic rings. Isoprenaline-mediated relaxations were attenuated in hypertonic (341+/-0.4 mOsmol) (mean+/-S.E.M.) solution. The concentration-response curve to isoprenaline was displaced to the right. The EC(50) (0.16+/-0.05 vs. 1.14+/-0.5 microM) significantly (n=6; P<0.05) increased without any changes to the maximum response. Hypertonic solution also attenuated methacholine-mediated relaxations resulting in a significant increase in the EC(50) (0. 28+/-0.04 vs. 0.52+/-0.04 microM) and reduced the maximal response (73+/-5% vs. 51+/-8%). In contrast, an increase in tonicity did not have any influence on sodium nitroprusside, forskolin or pinacidil concentration-response curves. Hypertonic solution also did not affect either basal cyclic AMP or cyclic GMP production. In addition, an increase in osmolarity did not affect isoprenaline-stimulated increases in the levels of cyclic AMP. However, an increase in the tonicity of Krebs solution significantly inhibited methacholine-stimulated (58%-34%) accumulation of cyclic GMP. The present data indicated that an increase in the tonicity of Krebs solution impaired endothelium-dependent relaxation and the associated increase in cyclic GMP production without affecting basal levels of this nucleotide. The inhibitory effects of high osmolarity on beta-adrenoceptor-mediated relaxation did not appear to be due to a reduction in cyclic AMP generation, or the result of inhibition of pinacidil-sensitive K(ATP)(+) channels. Moreover, an increase in the tonicity of Krebs solution did not influence relaxation induced by direct activation of adenylate cyclase or guanylate cyclase by forskolin and sodium nitroprusside, respectively.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

A remark on the high-conductance calcium-activated potassium channel in human endothelial cells

The patch-clamp technique was used to examine the presence of large conductance calcium-activated potassium channels (BKCa) in human endothelial cells and to characterize their properties in terms of voltage dependence, ion conduction and blockade by iberiotoxin (IbTX). Experiments were performed using cell-attached and outside-out configurations on human umbilical vein endothelial cells (HUVEC). For the experiments HUVECs, which were passaged 6-19 times, were used. In early passages channel activities were absent suggesting the appearance of BKCa depending on cell culture time. The inverse logarithmic voltage sensitivity was 10.17 mV (median) for cell-attached recordings and 12.10 mV (median) for outside-out patches (membrane voltage range: 60-120 mV, symmetrical 140 mM K+ solutions). The I/V relationship was quasilinear in the range of 0-80 mV and exhibited a nonlinear behaviour under further depolarization suggesting some kind of saturation mechanism. Using a sigmoid function to fit the data, channel conductance was calculated as 172.9 pS (median) for cell-attached patches and as 262.1 pS (median) for outside-out patches. IbTX, known as one of the most selective blockers of BKCa was perfused to outside-out patches. In two out of three experiments there was complete block of the ion channel after 1 min.

Vinblastine enhancement of hyposmosis-induced catecholamine release in cultured adrenal chromaffin cells: lack of relation to cell swelling and microtubule disruption

Exposure of chromaffin cells to hyposmotic solution has been shown to cause catecholamine release through the elevation of intracellular Ca2+ level. While cell volume change observed under hyposmotic conditions has been shown to be accompanied by the movement of various ions and suggested to be associated with the reorganization of cytoskeletons. In the present study, the effects of cytoskeleton-disrupting agents on hyposmosis-induced catecholamine release were examined to investigate a possible relationship between catecholamine release and cell volume change under hyposmotic conditions. Hyposmosis-induced catecholamine release was enhanced by pre-treatment of the cells with a microtubule-disrupting agent vinblastine, but not significantly altered by a microfilament-disrupting agent cytochalasin B. Vinblastine also caused an additional increase in the intracellular Ca2+ but failed to affect the cell volume change under hyposmotic conditions. In contrast, the hyposmosis-induced release was not significantly altered by either colchicine, another microtubule-disrupting agent, or taxol, a microtubule-stabilizing agent. These results indicate that vinblastine enhances hyposmosis-induced catecholamine release through an additional increase in the intracellular Ca2+ and furthermore suggest that this effect of vinblastine on the hyposmosis-induced release is unassociated with the disruption of the microtubule system, providing evidence for a lack of the direct relationship between catecholamine release and the cell volume change observed under hyposmotic conditions.

Regulatory volume decrease and associated osmolyte fluxes in cerebellar granule neurons are calcium independent

To investigate a possible role for Ca as a transduction signal for regulatory volume decrease (RVD), the effects of external Ca removal, Ca channel blockers (Cd, Co, La, Gd, verapamil, diltiazem, dihydropyridines) and inhibitors of endoplasmic reticulum Ca release (dantrolene, ryanodine, TMB-8) were examined on RVD and on the swelling-activated efflux of two main osmolytes: Cl (traced by 125I) and [3H]taurine. Omission of Ca plus EGTA did not affect RVD or osmolyte release but when BAPTA was the chelator, RVD decreased 20%, 125I fluxes were unaffected and taurine stimulated efflux decreased (20%) while the basal efflux slightly increased (<10%). Verapamil, diltiazem, Co, Cd, La and Gd did not affect RVD or osmolyte fluxes. Nimodipine and nitrendipine (25-50 microM) markedly decreased RVD and osmolyte fluxes (>90%) through a mechanism independent of extracellular Ca. Swelling elicited an increase in cytosolic Ca measured by fura-2, which was notably variable ranging 50-350 nM. However, RVD and osmolyte fluxes were not affected by the blockers of endogenous Ca release dantrolene, ryanodine and TMB-8 or by the permeable Ca chelator BAPTA-AM, even when the cytosolic Ca increase was abolished by the chelator. These results indicate that 1) RVD and osmolyte fluxes are independent of extracellular Ca 2) RVD, osmolyte release and cytosolic Ca raise are only coincident events. Consequently, Ca is unlikely to be a transducing signal for RVD in neurons.